1. Field of the Invention
The present invention relates to an oscillation apparatus formed by a ring oscillator including an odd number of inverters (more than two inverters) connected in a ring, each of the inverters having one drive MOS transistor and one load MOS transistor.
2. Description of the Related Art
Generally, ring oscillator incorporating oscillation apparatuses have been used in a high frequency superposing module of an optical pickup laser diode driver for reading information from an optical disk such as a digital versatile disk (DVD) or a MiniDisk (MD). In such a module, a high frequency current of several hundreds of MHz is superposed onto a laser diode drive current to reduce various laser noises, which requires a stabilization of the oscillation frequency.
In a prior art oscillation apparatus formed by a ring oscillator including an odd number of inverters (more than two inverters) connected in a ring, each of the inverters having one drive MOS transistor and one load MOS transistor, a constant current generating circuit is adapted to generate load currents, so that each of the load currents flows through the load MOS transistor of one of the inverters (see: JP-2003-283305A). This will be explained later in detail.
In the above-described prior art oscillation apparatus, however, when the threshold voltages of the drive transistors fluctuate due to the manufacturing steps such as the impurity implantation and diffusion steps, the oscillation frequency would fluctuate.